This invention is directed to adhesive and sealant compositions which contain dissimilar arm asymmetric radial block copolymers. More particularly, the invention is related to such compositions containing asymmetric block copolymers which have both hydrogenated and unhydrogenated sets of arms.
Block copolymers have been employed in adhesive compositions for many years, primarily because of their high cohesive strengths and their ability to crosslink without a chemical vulcanization step. Block copolymers such as those described in U.S. Pat. No. 3,239,478 are either linear or radial or star styrene-butadiene or styrene-isoprene block copolymers. The high cohesive strength of these polymers is often a detrimental quality in certain applications. In the past, cohesive strength was reduced by adding a styrene-isoprene diblock copolymer to the primary block copolymer to lower the cohesive strength and give less elasticity and better conformability. U.S. Pat. No. 4,391,949 suggested another approach whereby a star-shaped asymmetric block copolymer having styrene-diene and diene homopolymer arms was used.
These conventional block copolymers when used in adhesives tend to degrade in processing and/or over time because they are unsaturated in the main rubber chain. These unsaturation sites are reactive sites which are vulnerable to attack, such as by free radicals created by oxidation, ultraviolet light or mechanical action. As a result, the polymer chain may be severed by chain scission, reducing the molecular weight and those properties which are sensitive to molecular weight. Alternatively, the unsaturation sites may be subjected to grafting and crosslinking reactions which raise the molecular weight and undesirably stiffen the polymer making it unprocessable or ineffective as an adhesive. Hydrogenating the conventional unsaturated base polymers creates a nonpolar polymer which, although more stable, is difficult to tackify with resin additives and which is therefore inferior to conventional polymers in some applications, including pressure sensitive adhesives.
The present invention offers a solution to some of these problems without sacrificing the adhesive qualities of unsaturated block copolymers. It does so by providing a polymer which has both unsaturated and saturated arms.